A German airline has threatened pilots with two years in jail if they secretly film stewardesses having sex on planes or pass the videos around.

Holiday carrier Condor Airlines was forced into the move after bosses heard rumours that its pilots were passing round videos and pictures of stewardesses in sex acts with other staff on planes or other company sites.

The images were being bartered like trophies between pilots on the airline, which is owned by British travel giant Thomas Cook.

This behaviour could lead to dismissal and a "criminal" offence if a flight is endangered of up to two years in prison, the firm warned on an internal website in June.

"We have learned of the following facts: within the circle of the pilots, there are obviously some colleagues secretly taking photos and/or video recordings of the female colleagues who show the affected persons in clear sexual acts," the website said, according to German news magazine Focus.

However, although the airline has launched an investigation, it has not yet been able to unearth any staff sex videos. It is understood this is the first time the 61-year-old carrier has ever had to issue such a warning.

The firm said: "We are rigorously pursuing any allegation. We will of course initiate disciplinary and legal measures if we find that there is any substance to any kind of claim.

"We have informed our employees of the rumours and reminded them of our strict code of conduct which we expect all employees to adhere to at all time."

Condor was launched in 1956 and flies to 80 destinations across Europe, America, Africa and Asia.